A generator system for a gas turbine engine, such as that found in aircraft, ships, and some terrestrial vehicles, may include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG includes permanent magnets on its rotor. When the PMG rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a generator control device, which in turn outputs a DC current. This DC current next is provided to stator windings of the exciter. As the rotor of the exciter rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the rotor of the exciter rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main generator. Finally, as the rotor of the main generator rotates, three phases of AC current are typically induced in its stator, and this three-phase AC output can then be provided to a load such as, for example, an aircraft, ship, or vehicle electrical system.
Because some aircraft generators have potential rotational speeds up to and in excess of 24,000 rpm, potentially large centrifugal forces may be imposed upon the rotors in generators during operation. Given these potentially stressful operating conditions, the rotors are carefully designed and manufactured, so that the rotors are precisely balanced and reliable. Improper balancing not only can result in inefficiencies in the operation of a generator, but may also affect the reliability of the generator.
Among the components that can affect reliability and proper balancing of the rotors are the wire coils wound on the rotor. The centrifugal forces experienced by a rotor may be strong enough to cause bending of the wires of these coils so that the wires then extend into what is known as the interpole region. Over time, such bending can result in mechanical breakdown of the wires and compromise of the coil insulation system. Additionally, because the coils are assemblies of individual wires that can move to some extent with respect to one another and with respect to the remaining portions of the rotors, the coils are a potential source of imbalance within the rotor, which can lead to reduced reliability and can potentially compromise the insulation system. Even asymmetrical movements of these coils on the order of only a few thousandths of an inch can, in some instances, be significant to the performance of the generator.
In order to improve the strength and reliability of the wire coils and the coil insulation system, and to minimize the amount of imbalance in the rotors that may occur due to the wire coils, the rotors may include a coil retention system. With a coil retention system, substantially rigid interpole wedges are inserted in between neighboring poles of the rotors to reduce the likelihood of coil wire bending or movement. The interpole wedges, which can be constructed of high strength, lightweight metals, are held in place by interpole retainer wedges. The interpole retainer wedges, which can be constructed of titanium, or other high-strength, lightweight metals, are typically located between an upper surface of neighboring rotor coils and the pole tips of the neighboring poles.
The mass of the coil and the coil retention system results in a centrifugal force being developed as the rotor rotates these components. With the above-described coil retention system configuration, this centrifugal force is restrained by the pole tips. The overall centrifugal force magnitude that is developed during rotor rotation is influenced by various factors, including the rotational mass, the radius of rotation of the rotational mass, and the rotational speed. In recent years, the demand for lower weight, higher power generators has increased, which has resulted in generators with rotors that rotate faster and have larger diameters than previously designed rotors. As just noted, both of these factors tend to increase the centrifugal loading on the coil retention system.
Although presently designed coil retention systems are capable of providing the above-described benefits at relatively lower rotational rotor speeds, the design of these conventional retention systems limits their effectiveness. In particular, the loading on the conventional retention system components may exceed the structural capabilities of the components at relatively high rotor rotational speeds. Although stronger metallic materials could be used, this may increase the weight and/or size and/or cost of the generator. Moreover, while some lightweight composite materials have been used in some portions of the generator, the present inventors did not consider these to have sufficient strength to be used in the coil retention system.
Hence, there is a need for a coil retention system for use in generators that can withstand the centrifugal loads imposed at relatively high rotor rotational speeds and/or does not significantly increase generator weight and/or size and/or cost. The present invention addresses one or more of these needs.